On July 9, 1962, U.S. military researchers on a tiny Pacific atoll called Johnston Island fired a thermonuclear weapon into outer space. Code-named Starfish Prime, the launch onboard a Thor ballistic missile was the latest of a series of similar classified tests the U.S. Defense Department had begun four years before. But as the rocket rose on its smoky plume, few on the launch team realized that the forthcoming 1.4-megaton orbital burst was to yield surprising long-term results.

Hotel operators in Hawaii, some 1,300 kilometers away, were expecting a good show, though. Word had leaked of this latest "rainbow bomb" test shot, so a few enterprising resorts had organized rooftop parties from which guests could better view the distant fireworks. When the warhead detonated that evening at an altitude of 400 kilometers, it produced a brilliant white flash that momentarily lit up sea and sky like a noonday sun. Then, for about a second, the heavens turned light green.

This article was originally published with the title Nuclear Explosions in Orbit.

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